Enrollment of participants Sample size estimates There is scarcity of research indicating the seroconversion rates to OPV among infants residing in a rural population of Pakistan. The investigators assumed that a conservative estimate of seroconversion to OPV among infants till 14 weeks of age for a rural population of Pakistan would be about 50%. These seroconversion rates are consistent with the lower limits of OPV seroconversion rates observed among breastfed young infants in one solitary study on the subject in Pakistan. To detect a 20% improvement in seroconversion rates upon zinc administration, the investigators would require a minimum of 242 subjects (121 subjects occupying each group) at 5% level of significance and 90% power. Estimating a 30% maximum dropout or refusal rate in comparable studies involving serological testing, the investigators would require a total of 320 subjects, with 160 subjects occupying each group.